Hitherto, in a field of a gasoline-type internal combustion engine using gasoline as a fuel, spark ignition combustion for forcibly combusting a mixture by means of spark ignition by an ignition plug is generally adapted. However, in recent years, in place of the spark ignition combustion, research in employment of so-called compression self-ignition combustion in the gasoline-type internal combustion engine is in progress. The compression self-ignition combustion is combustion by self-ignition in a high-temperature and high-pressure environment generated by compressing a mixture by a piston.
The compression self-ignition combustion is such combustion that the mixture self-ignites at a plurality of positions simultaneously in the combustion chamber, and it is said that a combustion period is shorter and a higher heat efficiency is provided compared to the spark ignition combustion in which the combustion gradually extends through flame propagation. It should be noted that the compression self-ignition combustion is different from diffusion combustion in a diesel-type internal combustion engine using a diesel fuel, and is thus also referred to as homogeneous charge compression ignition (HCCI).
As a variable combustion system for an internal combustion engine, which is configured to simultaneously use the spark ignition combustion and the compression self-ignition combustion, for example, one described in Japanese Patent Application Laid-open No. 2013-227941 (Patent Document 1) is known. In the variable combustion system described in Japanese Patent Application Laid-open No. 2013-227941 (Patent Document 1), an internal combustion engine increased in a mechanical compression ratio (=geometrical compression ratio) up to approximately 18 is used to introduce an internal exhaust gas recirculation (EGR), to thereby realize compression self-ignition combustion excellent in fuel efficiency in a low load region. In a high load region in which the compression self-ignition combustion results in abnormal combustion, the combustion is switched to the spark ignition combustion, to thereby realize rapid combustion by carrying out high-pressure retard ignition for injecting a high-pressure fuel in a second half period in the compression stroke, and to suppress occurrence of knocking in the spark ignition combustion caused by the high mechanical compression ratio.
In Japanese Patent Application Laid-open No. 2013-227941 (Patent Document 1), the fuel is injected at a very high injection pressure as high as 30 MPa or more, and at a significantly retarded timing as late as the second half or later of the compression stroke (such as BTDC 20° to 0° CA). This injection at the high pressure and the retarded timing leads to reduction of an existing period of an unburnt mixture, and thus to avoidance of abnormal combustion.